Laser marking methods have recently been developed for marking metals, plastics, ceramics and glasses. Laser marking of metals typically involves a vaporization process, wherein a laser is used to remove or ablate metal from the surface along the travel path of the laser. The resultant marking comprises engraved or indented portions which provide three-dimensional contrast to the surface of the metal. Alternatively, laser marking of metals may be achieved by annealing a selected portion of the metal surface to provide areas of contrasting color. In this case, instead of removing metal from the surface, the laser is used to heat the surface of the metal to an annealing temperature which typically results in darkening of the annealed regions.
Plastics are typically laser marked by either changing the color of the plastic or engraving the surface of the plastic along the travel path of the laser. The color of the plastic is typically changed by localized melting and re-solidification of the plastic. In contrast, engraving is achieved by vaporization and removal of the plastic. Plastic laser engraving methods can be used to remove a surface layer of the plastic to reveal an underlying layer of contrasting color. Such a process is disclosed in U.S. Pat. No. 5,061,341 to Kildal et al.
Laser marking of ceramics and glasses has also been investigated, as a replacement for conventional etching, engraving and glazing techniques. For example, laser marking of glass has been achieved by ablation techniques as disclosed in U.S. Pat. No. 4,327,283 to Heyman et al. and U.S. Pat. No. 4,515,867 to Bleacher et al. In the disclosed methods, two coating layers are applied to a glass substrate, and the top layer is removed by the laser to reveal the contrasting underlayer.
Another technique for laser marking ceramics and glasses is disclosed in U.S. Pat. No. 4,769,310 to Gugger et al. and U.S. Pat. No. 5,030,551 to Herren et al. In this technique, a glaze having a radiation-sensitive additive comprising an inorganic pigment or titanium dioxide is deposited and fired on the surface of a ceramic or glass substrate. A laser beam is then used to irradiate the fired surface layer to thereby change the color of the surface layer in the areas of irradiation.
Each of the patents cited above is incorporated herein by reference.
Despite the above-noted marking techniques, a need still exists for a method of marking substrates such as metals, ceramics, glasses and plastics which is versatile and can be performed quickly, and which produces permanent marks of high resolution and contrast with minimal or no damage to the substrate.